1. Field of the Invention
The present invention relates to a lock-up control apparatus for an automatic transmission.
2. Description of the Related Art
Up to now, in a vehicle provided with a torque converter, at the time of travelling, by applying a lock-up clutch (hereinafter, which will be also referred to as “LC”) of the torque converter in a predetermined operation area, engine revolutions are suppressed to a low level, and an upgrade in fuel economics (fuel consumption) for the vehicle is implemented. Also, in recent years, in addition to an improvement in the practical fuel economics, as a purpose of providing a direct travelling feeling with respect to a press-down on an accelerator to a driver who is a user, an expansion of an application area of the lock-up clutch is implemented.
However, by expanding the application area of the lock-up clutch, as the frequency of shift transmissions for the automatic transmission is increased in an applied state of the lock-up clutch, a problem of a deterioration in the merchantability for a vehicle due to a shift transmission shock occurs. In particular, in the case of a down shift at a time when an acceleration pedal is strongly pressed down (kick-down shift transmission, which will be hereinafter also referred to as “KD shift transmission”), an engine torque is sharply increased. Also, as this is a shift transmission to a lower gear, the deterioration from the shift transmission shock is still more easily caused.
In order to solve such problems, a lock-up control apparatus for an automatic transmission is proposed for implementing an amelioration of the shift transmission shock by detecting a shift transmission instruction at the time of the kick-down, detecting a transition from a torque layer to an inertia layer after the detection, and gradually decreasing a clutch capacity of the lock clutch after the detection of this transition (lock-up control amount: for example, a duty ratio to a solenoid-operated valve corresponding to the lock-up clutch) (for example, see Japanese Unexamined Patent Application Publication No. 2005-321014).
Incidentally, along with the above-mentioned expansion of the application area of the lock-up clutch, while the lock-up clutch is applied, a shift transmission crossing a plurality of shift transmission gear speeds equal to or more than two gear speeds is also being generated in some cases. However, only a lock-up clutch capacity control similar to a normal KD shift transmission is performed with respect to such a KD shift transmission crossing two shift transmission gear speeds or more, and a special control has not been attempted.
In a case where the KD shift transmission crossing two gear speeds or more is generated, a necessary lock-up clutch capacity varies depending on whether the shift transmission control is a KD shift transmission for one gear speed each (hereinafter, which will be referred to as “continuous KD shift transmission”: for example, a continuous shift transmission control of a 5-4 gear speed shift transmission and a 4-3 gear speed shift transmission) or a KD shift transmission for performing a shift transmission while jumping over an intermediate shift transmission gear speed (in this example, fourth gear speed) (hereinafter, which will be referred to as “jumping KD shift transmission”: for example, a 5-3 gear speed shift transmission control).
To be more specific, at the time of the continuous KD shift transmission, after the first KD shift transmission control is ended, the next KD shift transmission control is started, and therefore the lock-up clutch capacity control similar to the normal KD shift transmission may be performed. However, at the time of the jumping KD shift transmission, the lock-up clutch capacity needs to be further lowered than that at the time of the continuous KD shift transmission. That is, in the KD shift transmission control in related art, when an intermediate shift transmission gear speed is passed through where a near neutral state is obtained and a transmission torque of the shift transmission clutch is small, a control of gradually increasing a control hydraulic pressure towards the normal lock-up clutch capacity along with an end of the first KD shift transmission control is once started, and thereafter the lock-up clutch capacity is decreased again. For that reason, the lock-up clutch capacity becomes excessive at the time of passing through the intermediate shift transmission gear speed, and problems occur in which the engine is retarded and the shift transmission shock is generated.